Nanomechanical behavior of individual phases and size effect in WC-Co by means of high temperature nanoindentation and electron microscopy: A study from ambient to high temperature

The dependence of the hardness and deformation mechanism of individual phases in WC-Co on microstructural parameters such as grain size and orientation was investigated by nanoindentation and electron microscopy from ambient to high temperature. At room temperature, the binder phase only exhibits a hardness of about 10 GPa, whilst the hardness of WC grains were measured about 29-30 and 37 GPa for the prismatic and basal orientation, respectively. All WC orientations exhibited a similar decrease in hardness as the temperature increased. A broad range of WC prismatic grain areas (AWC-prismatic), from about 2 to 1000 µm2, were selected and subsequently indented to investigate any size effect. A slight decrease in the hardness of WC prismatic grains (HWC-prismatic) as a function of AWC-prismatic was observed. Damage mechanisms occurring in WC-Co during nanoindentation were investigated for the different grain orientation at various temperature. The damage was visualised using electron microscopy near the residual indent as well as focused ion beam sectioning across the indent. The three dimensional distribution of plastic deformation across multiple grains in the vicinity of an indent was examined using Electron Back Scattered Diffraction (EBSD) and Electron Channelling Contrast Imaging (ECCI). The ECCI enabled the observation of crystal defects, especially dislocations, in th plastic zone. The dislocation density and spatial distribution in the deformed WC-Co were compared to that of an untested WC-Co to relate the quantity of defects as well as their origin to the state of stress in the material. The collected data represent useful guidance for manufacturer of hardmetals, provides important information underpinning an understanding of the relationship between WC-Co microstructure and mechanical properties, and also highlight the performance of WC-Co at operating temperatures. Please click Additional Files below to see the full abstract

Acoustofluidic measurements on polymer-coated microbubbles: primary and secondary Bjerknes forces

The acoustically-driven dynamics of isolated particle-like objects in microfluidic environments is a well-characterised phenomenon, which has been the subject of many studies. Conversely, very few acoustofluidic researchers looked at coated microbubbles, despite their widespread use in diagnostic imaging and the need for a precise characterisation of their acoustically-driven behaviour, underpinning therapeutic applications. The main reason is that microbubbles behave differently, due to their larger compressibility, exhibiting much stronger interactions with the unperturbed acoustic field (primary Bjerknes forces) or with other bubbles (secondary Bjerknes forces). In this paper, we study the translational dynamics of commercially-available polymer-coated microbubbles in a standing-wave acoustofluidic device. At increasing acoustic driving pressures, we measure acoustic forces on isolated bubbles, quantify bubble-bubble interaction forces during doublet formation and study the occurrence of sub-wavelength structures during aggregation. We present a dynamic characterisation of microbubble compressibility with acoustic pressure, highlighting a threshold pressure below which bubbles can be treated as uncoated. Thanks to benchmarking measurements under a scanning electron microscope, we interpret this threshold as the onset of buckling, providing a quantitative measurement of this parameter at the single-bubble level. For acoustofluidic applications, our results highlight the limitations of treating microbubbles as a special case of solid particles. Our findings will impact applications where knowing the buckling pressure of coated microbubbles has a key role, like diagnostics and drug delivery

On the damage and fracture of nuclear graphite at multiple length-scales

Gilsocarbon graphite, as a neutron moderator and load-bearing component in the core of the UK fleet of Advanced Gas-Cooled Reactors, possesses complex microstructural features including defects/pores over a range of length-scales from nanometres to millimetres in size. As a consequence, this material exhibits different characteristics when specimens of different length-scale are deformed. In this work, the deformation and fracture of this material have been characterised using in situ methods for specimens of micrometre size (meso-scale) and the results are then compared with those measured one length-scale smaller, and those at the macro-scale. At the micro-scale, sampling a volume of material (2x2x10 μm) excludes micro- and macro-size pores, the strength was measured to be as high as 1000 MPa (an elastic modulus of about 67 GPa). When the specimen size is increased by one order of magnitude to the meso-scale, the strength is reduced to about 100 MPa (an elastic modulus of about 20 GPa) due to the inclusion of micro-size pores. For larger engineering-size specimens that include millimetre-size pores the strength of the material averages about 20 MPa (an elastic modulus of about 11 GPa). This trend in the data is discussed and considered in the context of selecting the appropriate data for relevant multi-scale modelling

Interlaboratory Measurements of Contiguity in WC-Co Hardmetals

The contiguity of a hardmetal is a measure of the proportion of the carbide grain boundaries that are in direct contact with other carbide grain boundaries. Recent analysis of data available in the literature shows a large scatter in results and a significant difference in values measured from scanning electron microscope (SEM) images and from electron backscatter diffraction (EBSD) mapping. An interlaboratory exercise has been carried out with the measurement of a range of WC-Co hardmetal grades. For each grade, SEM images were acquired from both an etched surface and an ion beam polished surface and EBSD maps with two different processing routes. These maps and images were provided to the participants for measurement to eliminate variability from sample preparation and image acquisition. It was shown that measurement of contiguity from EBSD maps is likely to lead to an overestimation of contiguity, largely because EBSD maps do not have the resolution of SEM images to identify small binder phase regions between WC grains. Ion beam polishing combined with backscattered electron imaging was found to provide the best images of the microstructure to underpin a confident measurement of contiguity. However, high resolution SEM images of etched surfaces gave values close to those from ion beam polished samples so it is recommended that, as etching is much more widely available, high-resolution imaging of a lightly etched WC surface should be promoted as the preferred method for measurement of contiguity, in combination with backscattered imaging where possible. Even with good images, variation between operators can give uncertainties of approximately ±10%

Determination of Mechanisms of Abrasion in WC/Co hard metals by In situ micro‐tribology experiments

WC/Co Hardmetals make excellent wear resistant parts due to their combination of high strength and relatively good fracture resistance. Although the abrasion of these materials has been studied for many years, the mechanisms of damage are still not sufficiently understood to enable knowledge based optimisation of the materials. This paper reports results on experiments made on fine and coarse grained Hardmetals. The main results come in the form of videos or sequences of images showing damage accumulation. It was found that a major form of damage was fracture and fragmentation of the WC grains. These fragments combined with the binder phase but were eventually pushed out of the abraded area as the test proceeded. Larger fragments of material including whole grains were also removed from the surface. The videos of damage accumulation were supplemented by EBSD analysis of the samples in the early stages of damage, by measurements of the volume of the damage by topographical back-scattered electron imaging, and by FIB sectioning of samples after experiments to look at sub-surface damage

Real‐time in situ micro‐mechanical testing of hard metals using FIB‐SEM

The mechanical properties of WC/Co hardmetals were studied using in situ micro-mechanical characterisation techniques. Microscale pillars and cantilever beams were fabricated in different hardmetal microstructures using a focused ion beam microscope (FIB-SEM), to compare the behaviour between different orientation WC single crystals and polycrystalline structures. Micro-specimens in silicon were also tested to analyse the repeatability of the technique. The tests were done using a vacuum-compatible nanoindenter in a scanning electron microscope (SEM), which enabled simultaneous real-time observation of the test and acquisition of the load-displacement data. Please click Additional Files below to see the full abstract